(1) Field of the Invention
The present invention relates to a process for the preparation of a porous shaped article composed mainly of an organic polymer. More particularly, the present invention relates to an improvement in the process for preparing a porous shaped article by using a twin-screw extruder. Especially, the present invention relates to a process suitable for efficiently preparing a porous hollow filament or the like.
(2) Description of the Related Art
A porous hollow filament or porous film composed of a cellulose ester polymer or polysulfone polymer is often used for fluid treatments, for example, liquid treatments such as filtration, ultrafiltration, and dialysis, and gas treatments such as a separation of gases.
These porous products are prepared mainly by wet shaping processes such as wet spinning, melt shaping processes such as melt spinning, and plasticizing melt shaping processes using a plasticizer, such as plasticizing melt spinning.
For example, in the production of a porous hollow filament according to the wet spinning process, a spinning solution which is a mixed solution comprising an organic polymer such as a polysulfone, a solvent, and a pore-forming agent is prepared in a preparing tank, the solution is fed to a spinning nozzle, the spinning solution is extruded from the nozzle together with a coagulating liquid for a hollow portion, the extrudate is immersed in a coagulating bath to coagulate the liquid, and the solvent and pore-forming agent are removed by extraction.
Furthermore, in the case of the plasticizing melt spinning of a cellulose ester polymer or in the case of the plasticizing melt spinning for forming a porous hollow filament by using a poor solvent, a complicated process is adopted in which the cellulose polymer is first pulverized, the pulverized cellulose ester polymer is mixed and impregnated with a plasticizer, if necessary together with the poor solvent, by a blender or the like, the mixture is taken out and is heated and melt-extruded by a single-screw extruder, the extrudate is cooled and cue into a pelletized chip, the chip is dried by a hot air drier, the dried chip is heated and melted again, the melt is spun into a hollow filament by a spinning machine, the filament is cooled and solidified, the plasticizer or the plasticizer and poor solvent are removed by extraction with water or the like, and the formed porous yarn is wound.
In these conventional processes, a liquid to be supplied to a molding-extruding means such as a spinning nozzle must be prepared in advance through a preliminary step such as a mixing step or chip-forming step, and thus rationalization of the steps is necessary.
As the prior art reference disclosing a process for preparing a hollow yarn by a twin-screw extruder, there can be mentioned Japanese Unexamined Patent Publication No. 59-30825. According to this process, cellulose triacetate as a polymer is mixed with an organic solvent and a non-solvent over a period of 1 to 3 hours, the obtained powder impregnated with the solvent is transferred to a tank provided with a feed screw, the solvent-impregnated powder is fed from the tank to a feeding zone of an extruder, deaeration is effected during this feeding, the powder is kneaded and melted at 100.degree. to 220.degree. C., and the melt is compressed and extruded from a nozzle zone. This process, however, is disadvantageous in that a long time is required for the preliminary impregnation step, and that the influence of the affinity between the polymer and the solvent is strong at the impregnation step.